Modern motor vehicles are generally equipped with a catalyzer (catalytic converter) through which the exhaust gas from the internal combustion engine flows and which promotes chemical reactions between the exhaust-gas constituents which lead to the conversion of noxious exhaust-gas constituents into less harmful substances. However, the operating capability of the catalyzer decreases with increasing time in operation and, after a certain time in use, the catalyzer is no longer capable of reducing the exhaust emissions from the internal combustion engine to a sufficiently low level and must therefore be replaced. The time in operation after which replacement is required is heavily dependent on the conditions under which the catalyzer has to operate and cannot be predicted accurately. A catalyzer ages very rapidly, for example, when it is operated at very high temperatures. Moreover, if the quality of the fuel is poor or leaded fuel is used, the catalyzer may be subject to chemical poisoning, rendering the catalyzer completely or partially ineffective. In order to ensure low pollutant emissions over the entire life of the motor vehicle, the operating capability of the catalyzer should be monitored.
To assess the operating capability of the catalyzer, use can be made of the fact that heat is released in the conversion of the exhaust-gas constituents in the i catalyzer, giving rise to an increase in temperature. Owing to a high conversion rate, a fully operational catalyzer exhibits a large increase in temperature, i.e. the exhaust gas has a higher temperature at the outlet of the catalyzer than at the inlet of the catalyzer. The more limited the operating capability of the catalyzer, the lower is the increase in temperature.
German Patent Application No. DE 23 46 425 (corresponding to U.S. Pat. No. 3,882,451) describes a procedure in which the temperature at the outlet and at the inlet of the catalyzer is measured and the operating capability of the catalyzer is inferred from the temperature difference. If the temperature difference is too small, an alarm device is activated to indicate that the catalyzer should be replaced.